In recent years, a small electronic device represented by an LED attracts attention as an exposing device in an electrophotographic apparatus.
Although an LED or an OLED is small in size, the LED or the OLED includes an extraordinarily large number of light-emitting points compared with a laser optical system and the like. Moreover, since a Selfoc (registered trademark) lens array is used, the LED or the OLED include a large number of lenses. As a result, fluctuation in optical characteristics occurs in a main scanning direction. Because of fluctuation in the characteristics of the light-emitting points and the characteristics of the Selfoc lenses, respective beam profiles are different. Therefore, when a halftone image is printed, streak-like density unevenness (vertical steaks or streak unevenness) occurs.
In order to reduce the density unevenness in the halftone image, in general, correction processing called beam diameter correction is performed on the side of an exposing device such as an LED. However, the beam diameter correction has the opposite effect if conditions change. Therefore, for example, there is proposed a method of changing the intensity of dot diameter correction and performing the dot diameter correction that is stable against environmental changes and the like.
Even if such a method is adopted, in the dot diameter correction, in particular, if the distance between an LED and a photoconductive member deviates from a focal position, the distance may be unable to be adjusted to the focal position. Because of the characteristics of the Selfoc lens array, the focal distance is as very small as about several ten micrometers. Therefore, even if fluctuation in the distance between the LED and the photoconductive member is slight, the fluctuation affects beam profiles. To make the matter worse, since the characteristics of the respective light-emitting points are disordered, the effect of the dot diameter correction may be unable to be obtained.
The related art is disclosed in, for example, Japanese Patent No. 3214124.